When a building is connected to a municipal sewer line, a clean out pipe typically is installed in conjunction with the line leading from the building to the sewer to allow access to both for line maintenance. The access end of the clean out line typically is installed at or below grade. In the case of residential units, most jurisdictions require a separate clean out line for each unit. Thus, for multi-unit construction, such as townhomes, several clean out lines can be installed in a relatively small space. During construction of a building, heavy trucks and other equipment are often driven over ground containing one or more clean out lines and other utility lines, thereby raising the risk of damage to these lines. Consequently, there is a need for a system that can potentially provide some protection for and warning of the presence of an in-ground utility line, such as a sewer clean out line.